Microelectronic packages and methods therefor

ABSTRACT

A method of making a microelectronic assembly can include molding a dielectric material around at least two conductive elements which project above a height of a substrate having a microelectronic element mounted thereon, so that remote surfaces of the conductive elements remain accessible and exposed within openings extending from an exterior surface of the molded dielectric material. The remote surfaces can be disposed at heights from said surface of said substrate which are lower or higher than a height of the exterior surface of the molded dielectric material from the substrate surface. The conductive elements can be arranged to simultaneously carry first and second different electric potentials: e.g., power, ground or signal potentials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/012,949, filed on Jan. 25, 2011, which is a continuation of U.S.application Ser. No. 11/318,404, filed on Dec. 23, 2005, the disclosuresof which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to microelectronic packages and to methodsof making and testing microelectronic packages.

BACKGROUND OF THE INVENTION

Microelectronic devices such as semiconductor chips typically requiremany input and output connections to other electronic components. Theinput and output contacts of a semiconductor chip or other comparabledevice are generally disposed in grid-like patterns that substantiallycover a surface of the device (commonly referred to as an “area array”)or in elongated rows which may extend parallel to and adjacent each edgeof the device's front surface, or in the center of the front surface.Typically, devices such as chips must be physically mounted on asubstrate such as a printed circuit board, and the contacts of thedevice must be electrically connected to electrically conductivefeatures of the circuit board.

Semiconductor chips are commonly provided in packages that facilitatehandling of the chip during manufacture and during mounting of the chipon an external substrate such as a circuit board or other circuit panel.For example, many semiconductor chips are provided in packages suitablefor surface mounting. Numerous packages of this general type have beenproposed for various applications. Most commonly, such packages includea dielectric element, commonly referred to as a “chip carrier” withterminals formed as plated or etched metallic structures on thedielectric. These terminals typically are connected to the contacts ofthe chip itself by features such as thin traces extending along the chipcarrier itself and by fine leads or wires extending between the contactsof the chip and the terminals or traces. In a surface mountingoperation, the package is placed onto a circuit board so that eachterminal on the package is aligned with a corresponding contact pad onthe circuit board. Solder or other bonding material is provided betweenthe terminals and the contact pads. The package can be permanentlybonded in place by heating the assembly so as to melt or “reflow” thesolder or otherwise activate the bonding material.

Many packages include solder masses in the form of solder balls,typically about 0.1 mm and about 0.8 mm (5 and 30 mils) in diameter,attached to the terminals of the package. A package having an array ofsolder balls projecting from its bottom surface is commonly referred toas a ball grid array or “BGA” package. Other packages, referred to asland grid array or “LGA” packages are secured to the substrate by thinlayers or lands formed from solder. Packages of this type can be quitecompact. Certain packages, commonly referred to as “chip scalepackages,” occupy an area of the circuit board equal to, or onlyslightly larger than, the area of the device incorporated in thepackage. This is advantageous in that it reduces the overall size of theassembly and permits the use of short interconnections between variousdevices on the substrate, which in turn limits signal propagation timebetween devices and thus facilitates operation of the assembly at highspeeds.

Assemblies including packages can suffer from stresses imposed bydifferential thermal expansion and contraction of the device and thesubstrate. During operation, as well as during manufacture, asemiconductor chip tends to expand and contract by an amount differentfrom the amount of expansion and contraction of a circuit board. Wherethe terminals of the package are fixed relative to the chip or otherdevice, such as by using solder, these effects tend to cause theterminals to move relative to the contact pads on the circuit board.This can impose stresses in the solder that connects the terminals tothe contact pads on the circuit board. As disclosed in certain preferredembodiments of U.S. Pat. Nos. 5,679,977; 5,148,266; 5,148,265;5,455,390; and 5,518,964, the disclosures of which are incorporated byreference herein, semiconductor chip packages can have terminals thatare movable with respect to the chip or other device incorporated in thepackage. Such movement can compensate to an appreciable degree fordifferential expansion and contraction.

Testing of packaged devices poses another formidable problem. In somemanufacturing processes, it is necessary to make temporary connectionsbetween the terminals of the packaged device and a test fixture, andoperate the device through these connections to assure that the deviceis fully functional. Ordinarily, these temporary connections must bemade without bonding the terminals of the package to the test fixture.It is important to assure that all of the terminals are reliablyconnected to the conductive elements of the test fixture. However, it isdifficult to make connections by pressing the package against a simpletest fixture such as an ordinary circuit board having planar contactpads. If the terminals of the package are not coplanar, or if theconductive elements of the test fixture are not coplanar, some of theterminals will not contact their respective contact pads on the testfixture. For example, in a BGA package, differences in the diameter ofthe solder balls attached to the terminals, and non-planarity of thechip carrier, may cause some of the solder balls to lie at differentheights.

These problems can be alleviated through the use of speciallyconstructed test fixtures having features arranged to compensate fornon-planarity. However, such features add to the cost of the testfixture and, in some cases, introduce some unreliability into the testfixture itself. This is particularly undesirable because the testfixture, and the engagement of the device with the test fixture, shouldbe more reliable than the packaged devices themselves in order toprovide a meaningful test. Moreover, devices used for high-frequencyoperation are typically tested by applying high frequency signals. Thisrequirement imposes constraints on the electrical characteristics of thesignal paths in the test fixture, which further complicates constructionof the test fixture.

Additionally, when testing packaged devices having solder ballsconnected with terminals, solder tends to accumulate on those parts ofthe test fixture that engage the solder balls. This accumulation ofsolder residue can shorten the life of the test fixture and impair itsreliability.

A variety of solutions have been put forth to deal with theaforementioned problems. Certain packages disclosed in theaforementioned patents have terminals that can move with respect to themicroelectronic device. Such movement can compensate to some degree fornon-planarity of the terminals during testing.

U.S. Pat. Nos. 5,196,726 and 5,214,308, both issued to Nishiguchi etal., disclose a BGA-type approach in which bump leads on the face of thechip are received in cup-like sockets on the substrate and bondedtherein by a low-melting point material. U.S. Pat. No. 4,975,079 issuedto Beaman et al. discloses a test socket for chips in which dome-shapedcontacts on the test substrate are disposed within conical guides. Thechip is forced against the substrate so that the solder balls enter theconical guides and engage the dome-shaped pins on the substrate.Sufficient force is applied so that the dome-shaped pins actually deformthe solder balls of the chip.

A further example of a BGA socket may be found in commonly assigned U.S.Pat. No. 5,802,699, issued Sep. 8, 1998, the disclosure of which ishereby incorporated by reference herein. The '699 patent discloses asheet-like connector having a plurality of holes. Each hole is providedwith at least one resilient laminar contact extending inwardly over ahole. The bump leads of a BGA device are advanced into the holes so thatthe bump leads are engaged with the contacts. The assembly can betested, and if found acceptable, the bump leads can be permanentlybonded to the contacts.

Commonly assigned U.S. Pat. No. 6,202,297, issued Mar. 20, 2001, thedisclosure of which is hereby incorporated by reference herein,discloses a connector for microelectronic devices having bump leads andmethods for fabricating and using the connector. In one embodiment ofthe '297 patent, a dielectric substrate has a plurality of postsextending upwardly from a front surface. The posts may be arranged in anarray of post groups, with each post group defining a gap therebetween.A generally laminar contact extends from the top of each post. In orderto test a device, the bump leads of the device are each inserted withina respective gap thereby engaging the contacts which wipe against thebump lead as it continues to be inserted. Typically, distal portions ofthe contacts deflect downwardly toward the substrate and outwardly awayfrom the center of the gap as the bump lead is inserted into a gap.

Commonly assigned U.S. Pat. No. 6,177,636, the disclosure of which ishereby incorporated by reference herein, discloses a method andapparatus for providing interconnections between a microelectronicdevice and a supporting substrate. In one preferred embodiment of the'636 patent, a method of fabricating an interconnection component for amicroelectronic device includes providing a flexible chip carrier havingfirst and second surfaces and coupling a conductive sheet to the firstsurface of the chip carrier. The conductive sheet is then selectivelyetched to produce a plurality of substantially rigid posts. A compliantlayer is provided on the second surface of the support structure and amicroelectronic device such as a semiconductor chip is engaged with thecompliant layer so that the compliant layer lies between themicroelectronic device and the chip carrier, and leaving the postsprojecting from the exposed surface of the chip carrier. The posts areelectrically connected to the microelectronic device. The posts formprojecting package terminals that can be engaged in a socket orsolder-bonded to features of a substrate as, for example, a circuitpanel. Because the posts are movable with respect to the microelectronicdevice, such a package substantially accommodates thermal coefficient ofexpansion mismatches between the device and a supporting substrate whenthe device is in use. Moreover, the tips of the posts can be coplanar ornearly coplanar.

Despite all of the above-described advances in the art, still furtherimprovements in making and testing microelectronic packages would bedesirable.

SUMMARY OF THE INVENTION

In certain preferred embodiments of the present invention, a method ofmaking a microelectronic assembly includes providing a microelectronicpackage having a substrate, a microelectronic element overlying thesubstrate and at least one conductive element projecting from a surfaceof the substrate, the at least one conductive element having a surfaceremote from the surface of the substrate. The method desirably includescompressing the at least one conductive element so that the remotesurface thereof lies in a common plane, and after the compressing step,providing an encapsulant material around the at least one conductiveelement for supporting the microelectronic package and so that theremote surfaces of the at least one conductive element remainsaccessible at an exterior surface of the encapsulant material. Incertain preferred embodiments, the at least one conductive elementincludes at least two conductive elements.

In certain preferred embodiments, the at least two conductive elementsproject from a first surface of the substrate and the package furthercomprises at least two second conductive elements projecting from asecond surface of the substrate, the at least two second conductiveelements having surfaces remote from the second surface of thesubstrate. The method may also include compressing the at least twosecond conductive elements so that the remote surfaces of the at leasttwo second conductive elements lie in a common plane.

The substrate may be flexible, and may include a dielectric materialsuch as a polyimide. The microelectronic element is desirablyelectrically interconnected with the substrate, such as by usingconductive leads, wires or traces. The microelectronic element may be asemiconductor chip having a front face with contacts and a back faceremote therefrom. In certain preferred embodiments, the front face ofthe semiconductor chip faces the substrate. In other preferredembodiments, however, the front face of the semiconductor chip facesaway from the substrate and the back face of the semiconductor chipfaces the substrate. A compliant layer may be disposed between themicroelectronic element and the substrate.

In preferred embodiments, the at least two conductive elements aredisposed over a top surface of the substrate. In other preferredembodiments, the at least two conductive elements are disposed over abottom surface of the substrate. The substrate may include a pluralityof dielectric layers, and a plurality of layers of conductive traces mayextend through the substrate.

The method may include making a second microelectronic package using thesteps discussed above, and stacking the second microelectronic packageatop the first microelectronic package, the first and secondmicroelectronic packages being electrically interconnected togetherthrough the conductive elements.

In another preferred embodiment of the present invention, a method ofmaking a microelectronic assembly includes providing a mold having aninternal cavity, placing a microelectronic package into the cavity ofthe mold, the microelectronic package including a substrate, amicroelectronic element overlying the substrate and at least oneconductive element projecting from a first surface of the substrate, theat least one conductive element having a surface remote from the firstsurface of the substrate. The method desirably includes utilizing themold for compressing the at least one conductive element, and after thecompressing step and while the mold remains in contact with the remotesurface of the at least one conductive element, introducing anencapsulant material into the cavity of the mold for encapsulating themicroelectronic element and surrounding the at least one conductiveelement. The mold preferably remains in contact with the remote surfaceof the at least one conductive element during the introducing anencapsulant material step so that the remote surface of the at least oneconductive element remains accessible at an exterior surface of theencapsulant material. After the encapsulant material is introduced, itmay be cured.

In other preferred embodiments of the present invention, a method ofmaking a microelectronic assembly includes providing a mold having aninternal cavity, placing a microelectronic package into the cavity ofthe mold, the microelectronic package including a substrate, at leastone microelectronic element overlying the substrate and at least twovertical conductors projecting from a first surface of the substrate,the at least two vertical conductors having surfaces remote from thefirst surface of the substrate. The method desirably includes utilizingthe mold for compressing the at least two vertical conductors so thatthe remote surfaces of the two vertical conductors lie in a commonplane, and after the compressing step, introducing an encapsulantmaterial into the cavity of the mold for encapsulating the at least onemicroelectronic element and surrounding the at least two verticalconductors, wherein the mold remains in contact with the remote surfacesof the at least two vertical conductors during the introducing anencapsulant material step so that the remote surfaces of the at leasttwo vertical conductors remain accessible at an exterior surface of theencapsulant material. The vertical conductors may be conductive posts ormetallic masses.

In preferred embodiments, the substrate has a central region andperipheral regions adjacent outer edges of the substrate. The at leastone microelectronic element desirably overlies the central region of thesubstrate and the vertical conductors are disposed in the peripheralregions of the substrate. The method may include making a secondmicroelectronic package in accordance with the steps discussed above andstacking the second microelectronic package over the firstmicroelectronic package, with the first and second microelectronicpackages being electrically interconnected through the verticalconductors.

One aspect of the invention provides a microelectronic package includinga microelectronic element such as a semiconductor chip and a flexiblesubstrate spaced from and overlying a first face of the microelectronicelement. The package according to this aspect of the invention desirablyincludes a plurality of conductive posts extending from the flexiblesubstrate and projecting away from the microelectronic element, with atleast some of the conductive posts being electrically interconnectedwith said microelectronic element. Most preferably, the packageaccording to this aspect of the invention includes a plurality ofsupport elements disposed between the microelectronic element and saidsubstrate and supporting said flexible substrate over themicroelectronic element. Desirably, at least some of the conductiveposts are offset in horizontal directions parallel to the plane of theflexible substrate from the support elements. For example, the supportelements may be disposed in an array with zones of the flexiblesubstrate disposed between adjacent support elements, and the posts maybe disposed near the centers of such zones.

The offset between the posts and the support elements allows the posts,and particularly the bases of the posts adjacent the substrate, to moverelative to the microelectronic element. Most preferably, thearrangement allows each post to move independently of the other posts.The movement of the posts allows the tips of the plural posts tosimultaneously engage contact pads on a circuit board despiteirregularities in the circuit board or the package, such as warpage ofthe circuit board. This facilitates testing of the package using asimple test board, which may have substantially planar contacts, andavoids the need for specialized, expensive test sockets.

Most preferably, the flexible substrate overlies the front orcontact-bearing face of the microelectronic element. At least some ofthe support elements desirably are electrically conductive elements suchas solder balls. The conductive support elements may electricallyinterconnect at least some of the contacts of the microelectronicelement with at least some of the conductive posts. In preferred forms,this arrangement can prove low-impedance conductive paths between theposts and the microelectronic element, suitable for high-frequencysignal transmission. Most desirably, at least some of the posts areconnected to at least some of the contacts on the microelectronicelement by conductive support elements immediately adjacent to thoseposts. Preferably, conductive traces provided on the flexible substrateelectrically interconnect at least some of the conductive posts with atleast some of the conductive support elements. These traces may be veryshort; the length of each trace desirably is equal to the offsetdistance between a single post and a single support element.

A further aspect of the present invention provides a microelectronicassembly, which desirably includes a package as discussed above and acircuit panel having contact pads. Tips of the posts remote from theflexible substrate confront the contact pads and are electricallyconnected thereto, most preferably by electrically conductive bondingmaterial such as solder. As further discussed below, the assembly can becompact and highly reliable.

A further aspect of the invention provides a microelectronic package,which includes a microelectronic element and a flexible substrate spacedfrom and overlying said microelectronic element. The flexible substrateis supported above said front face of said microelectronic element sothat said substrate is at least partially unconstrained in flexure. Forexample, the flexible substrate may be supported by support elements asdescribed above, or by other means such as a continuous compliant layer.Here again, the package includes a plurality of conductive postsextending from the flexible substrate and projecting away from themicroelectronic element, the conductive posts being electricallyconnected to the microelectronic element. The conductive posts havebases facing toward the flexible substrate. The package according tothis embodiment of the invention desirably includes elements referred toherein as “focusing elements” disposed between the bases of at leastsome of the posts and the substrate and mechanically interconnecting thebases of the conductive posts with the substrate. The focusing elementsdesirably have smaller areas than the bases of the posts. As furtherdiscussed below, this arrangement facilitates flexing of the substrateand movement of the posts.

Yet another aspect of the invention provides methods of processingmicroelectronic packages. Method according to this aspect of theinvention desirably include the step of advancing a microelectronicpackage having a flexible substrate supported over a surface of amicroelectronic element and having electrically conductive postsprojecting from said substrate until tips of said posts engage contactpads on a test circuit panel and the substrate flexes so that at leastsome base portions of said posts adjacent said flexible substrate moverelative to the microelectronic element. In preferred methods accordingto this aspect of the present invention, movement of the bases of theposts contribute to movement of the tips, allowing the tips to engagecontact pads even where the contact pads themselves are not coplanarwith one another.

The method according to this aspect of the invention may include thefurther steps of maintaining the tips of the posts in contact with saidcontact pads and testing the package during the maintaining step, as bytransmitting signals to and from the package through the engaged contactpads and posts. The method may be practiced using a simple circuitpanel, with simple contact pads. The method may further includedisengaging the tips from the contact pads after testing, and may alsoinclude bonding the tips of the posts to electrically conductiveelements of a circuit panel after disengagement from the test circuitpanel. One aspect of the present invention, a provides a microelectronicpackage which includes a mounting structure, a microelectronic elementassociated with the mounting structure, and a plurality of conductiveposts physically connected to the mounting structure and electricallyconnected to the microelectronic element. The conductive posts desirablyproject from the mounting structure in an upward direction. At least oneof the conductive posts may be an offset post. Each offset postpreferably has a base connected to the mounting structure, the base ofeach offset post defining a centroid. As further explained below, wherethe base has a regular, biaxially symmetrical or point symmetrical shapesuch as a circle, the centroid is simply the geometric center of thebase. Each offset post also desirably defines an upper extremity havinga centroid, the centroid of the upper extremity being offset from thecentroid of the base in a horizontal offset direction transverse to theupward direction. When the package according to this aspect of theinvention is engaged with an external unit such as a test fixture,vertically directed contact forces are applied by the contact pads ofthe external unit. The contact forces applied to each offset post arecentered at the centroid of the upper extremity. The reaction forcesapplied by the mounting structure to the base of the post are centeredat the centroid of the base. Because these centroids are offset from oneanother, the forces applied to the post tend to tilt it about ahorizontal axis. Tilting of the post causes the upper extremity of thepost to wipe across the surface of the contact pad, which promotes goodcontact between the post and the contact pad. The mounting structuredesirably is deformable, so that the bases of the posts can moverelative to the microelectronic element in the tilting mode discussedabove. The mounting structure also may be arranged to deform so as topermit translational movement of the posts in a vertical direction,toward the microelectronic element. The movement of individual posts maydiffer, so that the tips of numerous posts can be engaged with numerouscontact pads even where the tips of the posts are not coplanar with oneanother, the contact pads are not coplanar with one another, or both,prior to engagement of the posts and contact pads.

The mounting structure may include a flexible substrate, which may haveconductive traces formed thereon for electrically interconnecting theposts with a microelectronic element. The flexible substrate may be agenerally sheet like substrate extending substantially in a horizontalplane, the substrate having a top surface and a bottom surface, theconductive posts projecting upwardly from the top surface. The flexiblesubstrate may also include a plurality of gaps extending through thesubstrate and defining a plurality of regions, different ones of theposts being disposed on different ones of the regions such as disclosedin commonly assigned U.S. patent application Ser. No. 10/985,119,entitled “MICRO PIN GRID WITH PIN MOTION ISOLATION,” filed on Nov. 10,2004, the disclosure of which is hereby incorporated herein byreference. The package may incorporate a support layer such as acompliant layer disposed between the flexible substrate and themicroelectronic element. In other embodiments, the package may include aplurality of support elements spaced apart from one another and disposedbetween the flexible substrate and the microelectronic element, thebases of the posts being spaced horizontally from the support elementsas described in greater detail in the co-pending, commonly assigned U.S.patent application Ser. No. 11/014,439, entitled “MICROELECTRONICPACKAGES AND METHODS THEREFORE,” filed on Dec. 16, 2004, the disclosureof which is hereby incorporated herein by reference.

The microelectronic element of the package preferably has faces andcontacts, the contacts being electrically interconnected with theconductive posts. In certain embodiments, the contacts are exposed at afirst face of the microelectronic element and the mounting structureoverlies the first face. In other embodiments, the contacts are exposedat a first face of the microelectronic element and the mountingstructure overlies a second, oppositely directed face of themicroelectronic element.

A still further aspect of the invention provides methods of makingmicroelectronic packages and elements of such packages. A methodaccording to this aspect of the invention desirably includes providing ablank made of a conductive material such as copper, applying a fluidunder pressure, desirably a liquid, to the blank to form at least oneconductive terminal in the blank, and providing electricalinterconnections to the at least one conductive terminal. The at leastone conductive terminal may be a conductive post. The method may alsoinclude heating the blank so as to make the blank more ductile duringthe forming operation.

The contacts of the microelectronic element are desirably accessible atthe first face of the microelectronic element. That is, the flexiblesubstrate overlies the front or contact-bearing face of themicroelectronic element. However, the microelectronic element may have asecond face opposite the first face and the contacts may be accessibleat the second face of the microelectronic element. The microelectronicpackage may also include conductive elements, such as conductive tracesprovided on said flexible substrate, for electrically interconnectingsaid conductive terminals and said microelectronic element.

In certain preferred embodiments of the present invention, an assemblyfor testing microelectronic devices includes a microelectronic elementhaving faces and contacts, a flexible substrate, such as a dielectricsheet, spaced from and overlying a first face of the microelectronicelement, and a plurality of conductive posts extending from the flexiblesubstrate and projecting away from the first face of the microelectronicelement. At least some of the conductive posts are desirablyelectrically interconnected with the microelectronic element. Theconductive posts may have a base facing toward the flexible substrate.The assembly may incorporate one or more of the features disclosed incommonly assigned U.S. Provisional Application Ser. No. 60/662,199,entitled “MICROELECTRONIC PACKAGES AND METHODS THEREFORE,” filed Mar.16, 2005 [TESSERA 3.8-429], the disclosure of which is herebyincorporated by reference herein.

The assembly also desirably includes a plurality of support elementsdisposed between the microelectronic element and the substrate. Thesupport elements desirably support the flexible substrate over themicroelectronic element, with at least some of the conductive postsbeing offset from the support elements. A compliant material may bedisposed between the flexible substrate and the microelectronic element.

In certain preferred embodiments, at least one of the conductive supportelements includes a mass of a fusible material. In other preferredembodiments, at least one of the conductive support elements includes adielectric core and an electrically conductive outer coating over thedielectric core. The support element may also be elongated, having alength that is greater than its width or diameter.

The microelectronic element may be a printed circuit board or a testboard used to test devices such as microelectronic elements andmicroelectronic packages. The first face of the microelectronic elementmay be a front face of the microelectronic element and the contacts maybe accessible at the front face. In certain preferred embodiments, atleast some of the support elements are electrically conductive. Theconductive support elements desirably electrically interconnect at leastsome of the contacts of the microelectronic element with at least someof the conductive posts. In certain preferred embodiments, the supportelements include a plurality of second conductive posts extending fromthe flexible substrate. The second conductive posts preferably projecttoward the first face of the microelectronic element, with at least someof the second conductive posts being electrically interconnected withthe first conductive posts. In certain preferred embodiments, a firstconductive post is electrically interconnected to a contact through asecond conductive post disposed immediately adjacent to the firstconductive post.

The conductive posts may be elongated, whereby the posts have a lengththat is substantially greater than the width or diameter of the posts.The support elements may be disposed in an array so that the supportelements define a plurality of zones on the flexible substrate, eachzone being bounded by a plurality of the support elements definingcorners of the zone, with different ones of the conductive posts beingdisposed in different ones of the zones. In preferred embodiments, onlyone of the conductive posts is disposed in each of the zones.

In another preferred embodiments of the present invention, amicroelectronic assembly includes a microelectronic element having facesand contacts, a flexible substrate spaced from and overlying a firstface of the microelectronic element, and a plurality of first conductiveposts extending from the flexible substrate and projecting away from thefirst face of the microelectronic element, at least some of theconductive posts being electrically interconnected with themicroelectronic element. The assembly also desirably includes aplurality of second conductive posts extending from the flexiblesubstrate and projecting toward the first face of the microelectronicelement, the second conductive posts supporting the flexible substrateover the microelectronic element, at least some of the first conductiveposts being offset from the second conductive posts.

In preferred embodiments, at least some of the second conductive postsare electrically conductive, the second conductive posts electricallyinterconnecting at least some of the contacts of the microelectronicelement with at least some of the first conductive posts. At least someof the first conductive posts may be connected to at least some of thecontacts by second conductive posts located immediately adjacent to thefirst conductive posts. The assembly may also include conductive tracesprovided on the flexible substrate, whereby the conductive traceselectrically interconnect at least some of the first conductive postswith at least some of the contacts on the microelectronic element. Incertain preferred embodiments, at least one of the conductive tracesextends between adjacent conductive posts.

In another preferred embodiment of the present invention, amicroelectronic assembly includes a microelectronic element having facesand contacts, and a flexible substrate spaced from and overlying a firstface of the microelectronic element, the flexible substrate havingconductive traces provided thereon. The assembly also desirably includesa plurality of conductive elements extending between the contacts of themicroelectronic element and the conductive traces for spacing theflexible substrate from the microelectronic element and for electricallyinterconnecting the microelectronic element and the conductive traces.The conductive elements may be elongated, conductive posts. Theconductive traces preferably have inner ends connected with theconductive elements and outer ends that extend beyond an outer perimeterof the microelectronic element. The outer ends of the conductive tracesare desirably movable relative to the contacts of the microelectronicelement. The assembly may also include an encapsulant material disposedbetween the microelectronic element and the flexible substrate.

In another preferred embodiment of the present invention, amicroelectronic assembly desirably includes a circuitized substratehaving metallized vias extending from a first surface of the substratetoward a second surface of the substrate. The assembly may also includea microelectronic package having conductive posts projecting therefrom,the conductive posts being at least partially inserted into openings ofthe metallized vias for electrically interconnecting the microelectronicpackage and the substrate. The microelectronic package may include amicroelectronic element having faces and contacts, a flexible substratespaced from and overlying a first face of the microelectronic element,and a plurality of support elements extending between themicroelectronic element and the flexible substrate for spacing theflexible substrate from the microelectronic element. The conductiveposts are preferably electrically interconnected with the contacts ofthe microelectronic element and are provided on a region of the flexiblesubstrate that is located outside a perimeter of the microelectronicelement. The conductive posts are desirably movable relative to thecontacts of the microelectronic element.

Assemblies in accordance with preferred embodiments of the presentinvention facilitate testing of microelectronic elements and packageshaving non-planar contacts and interfaces, and avoids the need forspecialized, expensive test equipment. In preferred methods according tothis aspect of the present invention, movement of the bases of theconductive posts contribute to movement of the tips of the posts,allowing the tips to engage opposing contact pads even where the contactpads themselves are not coplanar with one another.

As noted above, conductive traces may be provided on a flexiblesubstrate for electrically interconnecting at least some of the firstconductive posts with at least some of the second conductive posts.These traces may be very short; the length of each trace desirably isequal to the offset distance between a first conductive post and asecond conductive post. In preferred forms, this arrangement can provelow-impedance conductive paths between the posts and the microelectronicelement, suitable for high-frequency signal transmission.

In another preferred embodiment of the present invention, amicroelectronic assembly includes a bare chip or wafer having contactson a front face thereof. The bare chip or wafer is juxtaposed with aflexible substrate having conductive posts on a top surface thereof andconductive terminals on a bottom surface thereof. At least some of theconductive posts are not aligned with some of the conductive terminals.The conductive posts are preferably interconnected with the conductiveterminals. During assembly, the tip ends of the conductive post areabutted against the contacts of the chip or wafer for electricallyinterconnecting the chip or wafer with the conductive terminals on theflexible substrate. An encapsulant may be provided between thechip/wafer and the flexible substrate. Conductive elements such assolder balls may be provided in contact with the conductive terminals.The misalignment of the conductive terminals with the conductive postsprovides compliancy to the package and enables the conductive terminalsto move relative to the chip/wafer. In certain preferred embodiments,the conductive posts have an outer layer of gold that is presseddirectly against the chip contacts. In other preferred embodiments, theelectrical interconnection between the conductive posts and the contactsis formed using an anisotropic conductive film or an anisotropicconductive paste, whereby the conductive particles are disposed betweenthe conductive posts and the contacts. In another preferred embodimentof the present invention, the encapsulant for holding the chip/wafer andthe flexible substrate together includes a non-conductive film or paste.

These and other preferred embodiments of the present invention will bedescribed in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sectional view of a package according to oneembodiment of the invention.

FIG. 2 is a fragmentary plan view of the package shown in FIG. 1.

FIG. 3 is a diagrammatic elevational view depicting the package of FIGS.1-2 in conjunction with a test circuit panel during one step of a methodaccording to one embodiment of the invention.

FIG. 4 is a view similar to FIG. 3 but depicting a later stage of themethod.

FIG. 5 is a diagrammatic, idealized perspective view depicting a portionof the package shown in FIGS. 1-4.

FIG. 6 is a fragmentary sectional view depicting a portion of anassembly including the package of FIGS. 1-5.

FIG. 7A shows a front elevational view of a testing assembly during amethod of testing a microelectronic element, in accordance with onepreferred embodiment of the present invention.

FIG. 7B shows the testing assembly of FIG. 7A during a later stage oftesting the microelectronic element.

FIG. 8 shows a cross-sectional view of an assembly for testingmicroelectronic elements, in accordance with still further preferredembodiments of the present invention.

FIG. 9 shows a front elevational view of a microelectronic package, inaccordance with another preferred embodiment of the present invention.

FIG. 10 shows the package of FIG. 9 after the package has been placed ina mold, in accordance with certain preferred embodiments of the presentinvention.

FIG. 11 shows the package of FIGS. 9 and 10 after an overmold has beenformed on the package, in accordance with certain preferred embodimentsof the present invention.

FIG. 12 shows another view of the package shown in FIG. 11.

FIG. 13 shows a stack of microelectronic packages, in accordance withcertain preferred embodiments of the present invention.

FIG. 14 shows a top plate of a mold for making a microelectronicpackage, in accordance with certain preferred embodiments of the presentinvention.

FIG. 15 shows a microelectronic package and a mold for forming anovermold on the package, in accordance with certain preferredembodiments of the present invention.

FIG. 16 shows a partial front elevational view of a microelectronicpackage having an overmold, in accordance with certain preferredembodiments of the present invention.

FIG. 17 shows the package of FIG. 16 in a stacked assembly.

FIGS. 18 and 19 show a mold for compressing conductive elements on amicroelectronic package, in accordance with certain preferredembodiments of the present invention;

FIG. 20 shows a partial view of a microelectronic package, in accordancewith certain preferred embodiments of the present invention.

FIG. 21 shows the package of FIG. 20 in a stacked assembly.

FIGS. 22-24 show a method of making a microelectronic package, inaccordance with another preferred embodiment of the present invention.

FIG. 25 shows a microelectronic package, in accordance with anotherpreferred embodiment of the present invention.

FIG. 26 shows the package of FIG. 25 in a stacked assembly.

FIGS. 27-32 show a method of making a microelectronic package, inaccordance with certain preferred embodiments of the present invention.

FIG. 33 shows a top plan view of the microelectronic package shown inFIG. 30.

FIG. 34 shows another view of the package shown in FIG. 32.

FIG. 35 shows the package of FIG. 34 in a stacked assembly.

FIG. 36 shows an X-ray image of a section of the package shown in FIG.34.

FIGS. 37A shows a cross-sectional view of a section of the package shownin FIG. 34.

FIG. 37B shows a magnified view of a section of the package shown inFIG. 34.

FIG. 38 shows a microelectronic package, in accordance with anotherpreferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, in accordance with one preferred embodiment of thepresent invention, a microelectronic package 100 includes amicroelectronic element, such as a semiconductor chip 102, having afront or contact bearing face 104 and electrical contacts 106 exposed atface 104. A passivation layer 108 may be formed over the contact bearingface 104 with openings at contacts 106.

The microelectronic package 100 also includes conductive supportelements 110 such as solder balls in substantial alignment andelectrically interconnected with contacts 106. As best seen in FIG. 2,contacts 106 and support elements 110 are disposed in an array which inthis case is a rectilinear grid, having equally spaced columns extendingin a first horizontal direction x and equally spaced rows extending in asecond horizontal direction y orthogonal to the first horizontaldirection. Each contact 106 and support element 110 is disposed at anintersection of a row and a column, so that each set of four supportelements 110 at adjacent intersections, such as support elements 110 a,110 b, 110 c and 110 d, defines a generally rectangular, and preferablysquare, zone 112. The directions referred to in this disclosure aredirections in the frame of reference of the components themselves,rather than in the normal gravitational frame of reference. Horizontaldirections are directions parallel to the plane of the front surface 104of the chip, whereas vertical directions are perpendicular to thatplane.

The package also includes a flexible dielectric substrate 114, such as apolyimide or other polymeric sheet, including a top surface 116 and abottom surface 118 remote therefrom. Although the thickness of thedielectric substrate will vary with the application, the dielectricsubstrate most typically is about 10 μm-100 μm thick. The flexible sheet114 has conductive traces 120 thereon. In the particular embodimentillustrated in FIG. 1, the conductive traces are disposed on the bottomsurface 118 of the flexible sheet 114. In other preferred embodiments,however, the conductive traces 120 may extend on the top surface 116 ofthe flexible sheet 114, on both the top and bottom faces or within theinterior of the flexible substrate 114. Thus, as used in thisdisclosure, a statement that a first feature is disposed “on” a secondfeature should not be understood as requiring that the first feature lieon a surface of the second feature. Conductive traces 96 may be formedfrom any electrically conductive material, but most typically are formedfrom copper, copper alloys, gold or combinations of these materials. Thethickness of the traces will also vary with the application, buttypically is about 5 μm-25 μm. Traces 120 are arranged so that eachtrace has a support end 122 and a post end 124 remote from the supportend.

Electrically conductive posts or pillars 126 project from the topsurface 116 of flexible substrate 114. Each post 126 is connected to thepost end 124 of one of the traces 120. In the particular embodiment ofFIGS. 1 and 2, the posts 126 extend upwardly through the dielectricsheet 114, from the post ends of the traces. The dimensions of the postscan vary over a significant range, but most typically the height h_(p)of each post above the top surface 116 of the flexible sheet is about50-300 μm. Each post has a base 128 adjacent the flexible sheet 114 anda tip 130 remote from the flexible sheet. In the particular embodimentillustrated, the posts are generally frustoconical, so that the base 128and tip 130 of each post are substantially circular. The bases of theposts typically are about 100-600 μm in diameter, whereas the tipstypically are about 40-200 μm in diameter. The posts may be formed fromany electrically conductive material, but desirably are formed frommetallic materials such as copper, copper alloys, gold and combinationsthereof. For example, the posts may be formed principally from copperwith a layer of gold at the surfaces of the posts.

The dielectric sheet 114, traces 120 and posts 126 can be fabricated bya process such as that disclosed in co-pending, commonly assigned U.S.Provisional Patent Application Ser. No. 60/508,970, the disclosure ofwhich is incorporated by reference herein. As disclosed in greaterdetail in the '970 Application, a metallic plate is etched or otherwisetreated to form numerous metallic posts projecting from the plate. Adielectric layer is applied to this plate so that the posts projectthrough the dielectric layer. An inner or side of the dielectric layerfaces toward the metallic plate, whereas the outer side of thedielectric layer faces towards the tips of the posts. The dielectriclayer may be fabricated by coating a dielectric such as polyimide ontothe plate around the posts or, more typically, by forcibly engaging theposts with the dielectric sheet so that the posts penetrate through thesheet. Once the sheet is in place, the metallic plate is etched to formindividual traces on the inner side of the dielectric layer.Alternatively, conventional processes such as plating may form thetraces or etching, whereas the posts may be formed using the methodsdisclosed in commonly assigned U.S. Pat. No. 6,177,636, the disclosureof which is hereby incorporated by reference herein. In yet anotheralternative, the posts may be fabricated as individual elements andassembled to the flexible sheet in any suitable manner, which connectsthe posts to the traces.

As best appreciated with reference to FIG. 2, the support ends 122 ofthe leads are disposed in a regular grid pattern corresponding to thegrid pattern of the support elements, whereas the posts 126 are disposedin a similar grid pattern. However, the grid pattern of the posts isoffset in the first and second horizontal directions x and y from thegrid pattern of the support ends 122 and support elements 110, so thateach post 126 is offset in the −y and +x directions from the support end122 of the trace 120 connected to that post.

The support end 122 of each trace 120 overlies a support element 110 andis bonded to such support element, so that each post 126 is connected toone support element. In the embodiment illustrated, where the supportelements are solder balls, the bonds can be made by providing thesupport elements on the contacts 106 of the chip and positioning thesubstrate or flexible sheet 114, with the posts and traces alreadyformed thereon, over the support elements and reflowing the solder ballsby heating the assembly. In a variant of this process, the solder ballscan be provided on the support ends 122 of the traces. The process stepsused to connect the support ends of the traces can be essentially thesame used in flip-chip solder bonding of a chip to a circuit panel.

As mentioned above, the posts 126 are offset from the support elements110 in the x and y horizontal directions. Unless otherwise specifiedherein, the offset distance d_(o) (FIG. 2) between a post and a supportelement can be taken as the distance between the center of area of thebase 128 (FIG. 1) of the post and the center of area of the upper end132 (FIG. 1) of the support element 110. In the embodiment shown, whereboth the base of the post and the upper end of the support element havecircular cross-sections, the centers of area lie at the geometriccenters of these elements. Most preferably, the offset distance d_(o) islarge enough that there is a gap 134 (FIG. 2) between adjacent edges ofthe base of the post and the top end of the support element. Statedanother way, there is a portion of the dielectric sheet 114 in gap 134,which is not in contact with either the top end 132 of the supportelement or the base 128 of the post.

Each post lies near the center of one zone 112 defined by four adjacentsupport elements 110, so that these support elements are disposed aroundthe post. For example, support elements 110 a-110 d are disposed aroundpost 126A. Each post is electrically connected by a trace and by one ofthese adjacent support elements to the microelectronic device 102. Theoffset distances from a particular post to all of the support elementsadjacent to that post may be equal or unequal to one another.

In the completed unit, the upper surface 116 of the substrate orflexible sheet 114 forms an exposed surface of the package, whereasposts 126 project from this exposed surface and provide terminals forconnection to external elements.

The conductive support elements 110 create electrically conductive pathsbetween the microelectronic element 102 and the flexible substrate 114and traces 120. The conductive support elements space the flexiblesubstrate 114 from the contact bearing face 104 of microelectronicelement 102. As further discussed below, this arrangement facilitatesmovement of the posts 126.

Referring to FIG. 3, in a method of operation according to a furtherembodiment of the invention, a microelectronic package 100 such as thepackage discussed above with reference to FIGS. 1 and 2 is tested byjuxtaposing the conductive posts 126 with contact pads 136 on a secondmicroelectronic element 138 such as a circuitized test board. Theconductive posts 126A-126D are placed in substantial alignment with topsurfaces of the respective contact pads 136A-136D. As is evident in thedrawing figure, the top surfaces 140A-140D of the respective contactpads 136A-136D are disposed at different heights and do not lie in thesame plane. Such non-planarity can arise from causes such as warpage ofthe circuit board 138 itself and unequal thicknesses of contact pads136. Also, although not shown in FIG. 3, the tips 130 of the posts maynot be precisely coplanar with one another, due to factors such asunequal heights of support elements 110; non-planarity of the frontsurface 104 of the microelectronic device; warpage of the dielectricsubstrate 114; and unequal heights of the posts themselves. Also, thepackage 100 may be tilted slightly with respect to the circuit board.For these and other reasons, the vertical distances Dv between the tipsof the posts and the contact pads may be unequal.

Referring to FIG. 4, the microelectronic package 100 is moved toward thetest board 138, by moving the test board, the package or both. The tips130 of the conductive posts 126A-126D engage the contact pads 136 andmake electrical contact with the contact pads. The tips of the posts areable to move so as to compensate for the initial differences in verticalspacing Dv (FIG. 3), so that all of the tips can be brought into contactwith all of the contact pads simultaneously using only a moderatevertical force applied to urge the package and test board 138 together.In this process, at least some of the post tips are displaced in thevertical or z direction relative to other post tips.

A significant portion of this relative displacement arises from movementof the bases 128 of the posts relative to one another and relative tomicroelectronic element 100. Because the posts are attached to flexiblesubstrate 114 and are offset from the support elements 110, and becausethe support elements space the flexible substrate 114 from the frontsurface 104 of the microelectronic element, the flexible substrate candeform. Further, different portions of the substrate associated withdifferent posts can deform independently of one another.

An idealized representation of the deformation of a single region 112 ofsubstrate 114 is shown in FIG. 5. The support elements 110 disposed atthe corners of the region allow the central part of the region to benddownwardly toward the microelectronic element 102, allowing the base 128of post 126 to also move downward toward the microelectronic element.This deformation is idealized in FIG. 5 as a pure displacement of thepost and the center of the region in the vertical or z direction. Inpractice, the deformation of the substrate may include bending and/orstretching of the substrate so that the motion of the base may include atilting about an axis in the x-y or horizontal plane as well as somehorizontal displacement of the base, and may also include othercomponents of motion. For example, one portion of the region may bereinforced by a trace, and will tend to be stiffer than the otherportions of the region. Also, a particular post may be positionedoff-center in its region 112, so that the post lies closer to onesupport element, or to a pair of support elements, on one side of thepost. For example, post 126 a (FIG. 2) may be disposed closer to supportelements 110 a and 110 b than to support elements 110 c and 110 d. Therelatively small portion of the substrate between the post and supportelements 110 a and 110 b will be stiffer in bending than the relativelylarge portion of the substrate between the posts and support elements110 c and 110 d. Such non-uniformities tend to promote non-uniformbending and hence tilting motion of the posts. Tilting of the poststends to move the tips 130 toward the microelectronic element. Thesupport elements 110 at the corners of the individual regionssubstantially isolate the various regions from one another, so that thedeformation of each region is substantially independent of thedeformation of other regions of the substrate 114. Depending on theconfiguration of the posts, the posts 126 themselves may also flex orbuckle to some degree, which provides additional movement of tips 76 inthe vertical or z direction.

The independent displacement of the posts relative to one another allowsall of the post tips 130 to contact all of the contact pads 136 on thetest substrate. For example, the flexible substrate 114 in the vicinityof conductive post 126C flexes substantially more than the flexiblesubstrate in the vicinity of conductive post 126B. In turn, the flexiblesubstrate 114 in the vicinity of conductive post 126B flexessubstantially more than the flexible substrate in the vicinity ofconductive post 126A.

Because all of the post tips 130 can be engaged reliably with all of thecontact pads 136, the package can be tested reliably by applying testsignals, power and ground potentials through the test circuit board 138and through the engaged posts and contact pads. Moreover, this reliableengagement is achieved with a simple test circuit board 138. Forexample, the contact pads 136 of the test circuit board are simple,planar pads. The test circuit board need not incorporate specialfeatures to compensate for non-planarity or complex socketconfigurations. The test circuit board can be made using the techniquescommonly employed to form ordinary circuit boards. This materiallyreduces the cost of the test circuit board, and also facilitatesconstruction of the test circuit board with traces (not shown) in asimple layout compatible with high-frequency signals. Also, the testcircuit board may incorporate electronic elements such as capacitors inclose proximity to the contact pads as required for certainhigh-frequency signal processing circuits. Here again, because the testcircuit board need not incorporate special features to accommodatenon-planarity, placement of such electronic elements is simplified. Insome cases, it is desirable to make the test circuit board as planar aspracticable so as to reduce the non-planarity of the system and thusminimize the need for pin movement. For example, where the test circuitboard is highly planar a ceramic circuit board such as a polishedalumina ceramic structure, only about 20 μm of pin movement willsuffice.

The internal features of package 100 are also compatible withhigh-frequency signals. The conductive support elements, traces andposts provide low-impedance signal paths between the tips of the postsand the contacts 106 of the microelectronic element. Because each post126 is connected to an immediately adjacent conductive support element110, traces 120 are quite short. The low-impedance signal paths areparticularly useful in high-frequency operation, as, for example, wherethe microelectronic element must send or receive signals at a frequencyof 300 MHz or more.

After testing the microelectronic package 100 may be removed from thetest circuit board 138 and permanently interconnected with anothersubstrate such as a circuit panel 140 (FIG. 6) having contact pads 142,as by bonding the tips 130 of posts 126 to the contact pads of thecircuit panel using a conductive bonding material 144 such as a solder.The solder-bonding process may be performed using conventional equipmentcommonly used for surface-mounting microelectronic components. Thus, thesolder masses may be provided on the posts 126 or on the contact pads142, and may be reflowed after engaging the posts with the contact pads.During reflow, the surface tension of the solder tends to center theposts on the contact pads. Such self-centering action is particularlypronounced where the tips of the posts are smaller than the contactpads. Moreover, the solder 144 wets the sides of the posts to at leastsome extent, and thus forms a fillet encircling the tip of each post, aswell as a strong bond between the confronting surfaces of the posts andpads.

Moreover, the tips 130 of the posts 126 can move relative to themicroelectronic element 102 to at least some degree during service so asto relieve stresses arising from differential thermal expansion andcontraction. As discussed above in connection with the testing step, theindividual posts 126 can move relative to the microelectronic elementand relative to the other posts by flexure or other deformation ofsubstrate 114. Such movement can appreciably relieve stresses in thesolder bonds between the posts and the contact pads, which wouldotherwise occur upon differential thermal expansion or contraction ofthe circuit board 140 and microelectronic element 102. Moreover, theconductive support elements or solder balls 110 can deform to furtherrelieve stresses in solder masses 144. The assembly is highly resistantto thermal cycling stresses, and hence highly reliable in service.

An underfill material (not shown) such as an epoxy or other polymericmaterial may be provided around the tips of the posts and around thecontact pads, so as to reinforce the solder bonds. Desirably, thisunderfill material only partially fills the gap between the package 100and the circuit board 140. In this arrangement, the underfill does notbond the flexible substrate 114 or the microelectronic device to thecircuit board. The underfill only reinforces the posts at their jointswith the contact pads. However, no reinforcement is required at thebases of the posts, inasmuch as the joint between the base of each postand the associated trace is extraordinarily resistant to fatiguefailure.

The assembly is also compact. Some or all of the posts 126 and contactpads 142 are disposed in the area occupied by the microelectronicelement 102, so that the area of circuit board 140 occupied by theassembly may be equal to, or only slightly larger than, the area of themicroelectronic element itself, i.e., the area of the front surface 104of the microelectronic element 100.

The foregoing discussion has referred to an individual microelectronicelement. However, the package may include more than one microelectronicelement or more than one substrate. Moreover, the process steps used toassemble the flexible substrate, support elements and posts to the chipsmay be performed while the chips are in the form of a wafer. A singlelarge substrate may be assembled to an entire wafer, or to some portionof the wafer. The assembly may be severed so as to form individualunits, each including one or more of the chips and the associatedportion of the substrate. The testing operations discussed above may beperformed prior to the severing step. The ability of the packages tocompensate for non-planarity in a test board or in the wafer itselfgreatly facilitates testing of a large unit.

The substrate and traces may deform locally in regions surrounding theposts. These regions tend to deform upwardly, leaving concavities in thebottom surface of the substrate. The posts may have heads, and theseheads may be lodged partially or completely within the concavities. Tocontrol deformation of the substrate, the top surface of the substratemay be abutted against a die having holes aligned with locations whereposts are forced through the substrate. Such a die can also help toprevent delamination of the substrate and traces. In variants of theprocess, the traces may be disposed on the top or bottom surface of asingle-layer substrate. The resulting post-array substrate can beassembled with a microelectronic element to form a package as discussedabove, or can be used in any other microelectronic assembly where asmall post array is desirable. The assembly process allows selectiveplacement of posts. It is not essential to provide the lands and holesin the traces. Thus, posts can be placed at any location along anytrace. Moreover, the posts may be formed from essentially any conductivematerial. Different posts may be formed from different materials. Forexample, posts subject to severe mechanical loading can be formedentirely or partially from hard refractory metals such as tungsten,while other posts may be formed from softer metals such as copper. Also,some or all of the posts may be formed entirely or partially fromcorrosion-resistant metals such as nickel, gold or platinum.

The dielectric sheet, traces and posts may be fabricated by a processsuch as that disclosed in co-pending, commonly assigned U.S. ProvisionalPatent Application Ser. No. 60/508,970, the disclosure of which ishereby incorporated by reference herein. As disclosed in greater detailin the '970 Application, a metallic plate is etched or otherwise treatedto form numerous metallic posts projecting from the plate. A dielectriclayer is applied to this plate so that the posts project through thedielectric layer. An inner face of the dielectric layer faces toward themetallic plate, whereas the outer face of the dielectric layer facestowards the tips of the posts. The dielectric layer may be fabricated bycoating a dielectric such as polyimide onto the plate around the postsor, more typically, by forcibly engaging the posts with the dielectricsheet so that the posts penetrate through the sheet. Once the sheet isin place, the metallic plate is etched to form individual traces on theinner side of the dielectric layer. Alternatively, conventionalprocesses such as plating may form the traces or etching, whereas theposts may be formed using the methods disclosed in commonly assignedU.S. Pat. No. 6,177,636, the disclosure of which is hereby incorporatedby reference herein. In yet another alternative, the posts may befabricated as individual elements and assembled to the flexible sheet inany suitable manner, which connects the posts to the traces.

In the completed unit, the upper surface of the substrate or flexiblesheet forms an exposed surface of the package, whereas posts projectfrom this exposed surface and provide terminals for connection toexternal elements.

FIGS. 7A and 7B show a method of testing a microelectronic element 260using a testing assembly 220. The microelectronic element 260, such as asemiconductor chip, has a front face 262 including contacts 264accessible at the front face and a rear face 266 remote therefrom. Inorder to test the microelectronic element 260, the contacts 264 of themicroelectronic element are juxtaposed with the conductive posts 246 ofthe test board 222. The contacts 264A-264D are placed in substantialalignment with top surfaces 250 of the respective conductive posts246A-246D. As is evident in the drawing figure, the top surfaces266A-266D of the respective contact pads 264A-264D are disposed atdifferent heights and do not lie in the same plane. Such non-planaritycan arise from causes such as warpage of the microelectronic element 260itself and unequal thicknesses of contact pads 264. Also, although notshown in FIG. 7A, the tips 250 of the posts 246 may not be preciselycoplanar with one another, due to factors such as unequal heights ofsupport elements 230; non-planarity of the first surface 224 of the testboard 222; warpage of the dielectric substrate 232; and unequal heightsof the posts themselves. Also, the microelectronic element 260 may betilted slightly with respect to the test board. For these and otherreasons, the vertical distances Dv between the contacts 264 and the tipsof the posts 246 may be unequal.

Referring to FIG. 7B, the microelectronic element 260 is moved towardthe test board 222, by moving the test board, the microelectronicelement or both toward one another. The contacts 264 engage theconductive posts 246A-246D for making electrical contact with theconductive posts. The tips 250 of the posts 246A-246D are able to moveso as to compensate for the initial differences in vertical spacing Dv(FIG. 7A), so that all of the tips can be brought into contact with allof the contact pads simultaneously using with only a moderate verticalforce applied to urge the microelectronic element 260 and the test board222 together. In this process, at least some of the post tips 246A-246Dare displaced in the vertical or z direction relative to others of thepost tips.

A significant portion of this relative displacement arises from movementof the bases 248 of the posts relative to one another and relative totest board 220. Because the posts are attached to flexible substrate 232and are offset from the support elements 230, and because the supportelements space the flexible substrate 232 from the first surface 224 ofthe test board, the flexible substrate 232 can deform. Further,different portions of the substrate associated with different posts candeform independently of one another. As pressure is applied by contacts264 onto the posts 246, the support elements 230 allow region 268 offlexible substrate 232 to bend downwardly toward the test board 222,allowing the base 248 of post 246B to also move downward toward the testboard. This deformation is idealized in FIG. 7B as a pure displacementof the post and the center of the region in the vertical or z direction.In practice, the deformation of the substrate 232 may include bendingand/or stretching of the substrate so that the motion of the base mayinclude a tilting about an axis in the x-y or horizontal plane as wellas some horizontal displacement of the base, and may also include othercomponents of motion. For example, one portion of the region may bereinforced by a conductive trace (not shown), which will tend to bestiffer than the other portions of the region. Also, a particular postmay be positioned off-center in its region 268, so that the post liescloser to one support element 230, or to a pair of support elements, onone side of the post. For example, post 246 a may be disposed closer tosupport elements 230 a and 230 b than to support elements 230 c and 230d. The relatively small portion of the substrate between the post andsupport elements 230 a and 230 b will be stiffer in bending than therelatively large portion of the substrate between the posts and supportelements 230 c and 230 d. Such non-uniformities tend to promotenon-uniform bending and hence tilting motion of the posts. Tilting ofthe posts tends to move the tips 250 toward the test board 222. Thesupport elements 230 at the corners of the individual regionssubstantially isolate the various regions from one another, so that thedeformation of each region is substantially independent of thedeformation of other regions of the flexible, dielectric substrate 232.Depending on the configuration of the posts, the posts 246 themselvesmay also flex or buckle to some degree, which provides additionalmovement of tips 250 in the vertical or z direction.

Referring to FIG. 7B, the independent displacement of the posts 246relative to one another allows all of the contacts 264 of themicroelectronic element 260 to contact all of the post tips 250 on thetest board 222. For example, the flexible substrate 232 in the vicinityof conductive post 246B flexes substantially more than the flexiblesubstrate in the vicinity of conductive post 246C. In turn, the flexiblesubstrate 232 in the vicinity of conductive post 246C flexessubstantially more than the flexible substrate in the vicinity ofconductive post 246D.

Because all of the contacts 264 can be engaged reliably with all of thepost tips 250, the microelectronic element 260 can be tested reliably byapplying test signals, power and ground potentials through the testboard 222 and through the engaged contacts and posts.

The test circuit board can be made using the techniques commonlyemployed to form ordinary circuit boards. The test circuit board mayincorporate electronic elements such as capacitors in close proximity tothe contact pads as required for certain high-frequency signalprocessing circuits. The internal features of the microelectronicelement 260 are also compatible with high-frequency signals. Theconductive support elements 230, traces 238 and posts 246 providelow-impedance signal paths between the tips 250 of the posts and thecontacts 264 of the microelectronic element 260. Because each post 246is connected to an immediately adjacent conductive support element 230,traces 238 may be quite short. The low-impedance signal paths areparticularly useful in high-frequency operation, as, for example, wherethe microelectronic element must send or receive signals at a frequencyof 300 MHz or more.

After testing, the microelectronic element 260 may be removed from thetesting assembly 220 and packaged using an interposer such as acircuitized, dielectric film. The microelectronic package, such as aball grid array package, may be connected with contact pads on a circuitpanel using a conductive bonding material such as solder. Thesolder-bonding process may be performed using conventional equipmentcommonly used for surface-mounting microelectronic components. Thus, thesolder masses may be provided on the terminals of the microelectronicpackage, and may be reflowed after engaging the terminals with theconductive pads.

Referring to FIG. 8, in certain preferred embodiments of the presentinvention, a testing assembly 320 has a compliant material 370positioned between a flexible substrate 332 and a test board 322. Thecompliant material layer 370 preferably does not substantially restrictmovement of conductive posts 346. The compliant material desirablyprevents contaminants from entering the testing assembly 320. Merely byway of example, the compliant material 370 may be a gel, foam or thelike. Despite the presence of the compliant material, conductiveelements 330 still support the flexible substrate 332 to a substantialdegree.

As described above in earlier embodiments, the conductive posts are freeto move independently of other conductive posts so as to ensure reliablecontact between each conductive post and each conductive pad on a testboard. The tips of the conductive posts are able to move so as tocompensate for potential differences in vertical spacing so that all ofthe tips can be brought into contact with all of the conductive padssimultaneously using with only a moderate vertical force applied to urgea testable package and a test board together. In this process, at leastsome of the tips of the conductive posts are displaced in the verticalor z direction relative to others of the post tips. Further, differentportions of the flexible substrate associated with different conductiveposts can deform independently of one another. In practice, thedeformation of the substrate may include bending and/or stretching ofthe substrate so that the motion of the base may include a tilting aboutan axis in the x-y or horizontal plane as well as some horizontaldisplacement of the base, and may also include other components ofmotion.

The dimensions of the conductive posts can vary over a significantrange, but most typically the height of each post above the surface ofthe dielectric substrate is about 50-300 μm. Each post has a baseadjacent the dielectric substrate and a tip remote from the dielectricsubstrate. In certain preferred embodiments, the posts are generallyfrustoconical, so that the base and tip of each post are substantiallycircular. The bases of the posts typically are about 100-600 μm indiameter, whereas the tips typically are about 40-200 μm in diameter.The posts may be formed from any electrically conductive material, butdesirably are formed from metallic materials such as copper, copperalloys, gold and combinations thereof. For example, the posts may beformed principally from copper with a layer of gold at the surfaces ofthe posts.

In certain preferred embodiments, the conductive traces are disposed ona bottom surface of the dielectric layer. However, in other embodiments,the conductive traces may extend on the top surface of the dielectriclayer; on both the top and bottom faces or within the interior of thedielectric layer. Thus, as used in this disclosure, a statement that afirst feature is disposed “on” a second feature should not be understoodas requiring that the first feature lie on a surface of the secondfeature. The conductive traces may be formed from any electricallyconductive material, but most typically are formed from copper, copperalloys, gold or combinations of these materials. The thickness of thetraces will also vary with the application, but typically is about 5μm-25 μm.

The tips of the posts may not be precisely coplanar with one another,due to factors such as non-planarity of the front surface of themicroelectronic device; warpage of the dielectric substrate; and unequalheights of the posts themselves. Also, the package may be tiltedslightly with respect to the circuit board. For these and other reasons,the vertical distances between the tips of the posts and the contactpads may be unequal. The independent displacement of the posts relativeto one another allows all of the post tips to contact all of the contactpads on the test substrate.

Because all of the post tips can be engaged reliably with all of thecontact pads, the package can be tested reliably by applying testsignals, power and ground potentials through the test circuit board andthrough the engaged posts and contact pads. Moreover, this reliableengagement is achieved with a simple test circuit board. For example,the contact pads of the test circuit board are simple, planar pads. Thetest circuit board need not incorporate special features to compensatefor non-planarity or complex socket configurations. The test circuitboard can be made using the techniques commonly employed to formordinary circuit boards. This materially reduces the cost of the testcircuit board, and also facilitates construction of the test circuitboard with traces (not shown) in a simple layout compatible withhigh-frequency signals. Also, the test circuit board may incorporateelectronic elements such as capacitors in close proximity to the contactpads as required for certain high-frequency signal processing circuits.Here again, because the test circuit board need not incorporate specialfeatures to accommodate non-planarity, placement of such electronicelements is simplified. In some cases, it is desirable to make the testcircuit board as planar as practicable so as to reduce the non-planarityof the system and thus minimize the need for pin movement. For example,where the test circuit board is highly planar a ceramic circuit boardsuch as a polished alumina ceramic structure, only about 20 μm of pinmovement will suffice.

In certain preferred embodiments of the present invention, a particlecoating such as that disclosed in U.S. Pat. Nos. 4,804,132 and5,083,697, the disclosures of which are incorporated by referenceherein, may be provided on one or more electrically conductive parts ofa microelectronic package for enhancing the formation of electricalinterconnections between microelectronic elements and for facilitatingtesting of microelectronic packages. The particle coating is preferablyprovided over conductive parts such as conductive terminals or the tipends of conductive posts. In one particularly preferred embodiment, theparticle coating is a metallized diamond crystal coating that isselectively electroplated onto the conductive parts of a microelectronicelement using standard photoresist techniques. In operation, aconductive part with the diamond crystal coating may be pressed onto anopposing contact pad for piercing the oxidation layer present at theouter surface of the contact pad. The diamond crystal coatingfacilitates the formation of reliable electrical interconnectionsthrough penetration of oxide layers, in addition to traditional wipingaction.

As discussed above, the motion of the posts may include a tiltingmotion. This tilting motion causes the tip of each post to wipe acrossthe contact pad as the tip is engaged with the contact pad. Thispromotes reliable electrical contact. As discussed in greater detail inthe co-pending, commonly assigned application Ser. No. 10/985,126 filedNov. 10, 2004, entitled “MICRO PIN GRID ARRAY WITH WIPING ACTION”[TESSERA 3.0-375], the disclosure of which is incorporated by referenceherein, the posts may be provided with features which promote suchwiping action and otherwise facilitate engagement of the posts andcontacts. As disclosed in greater detail in the co-pending, commonlyassigned application Ser. No. 10/985,119 filed Nov. 10, 2004, entitled“MICRO PIN GRID WITH PIN MOTION ISOLATION” [TESSERA 3.0-376], thedisclosure of which is also incorporated by reference herein, theflexible substrate may be provided with features to enhance the abilityof the posts to move independently of one another and which enhance thetilting and wiping action.

Referring to FIG. 9, in accordance with another preferred embodiment ofthe present invention, a microelectronic package includes a substrate400 having a first or top surface 402 and a second or bottom surface404. The substrate includes conductive pads 406 accessible at the topsurface 402 and conductive posts 408 projecting from the second surface404 of the substrate 400. The substrate 400 may be flexible and may bemade of a dielectric material such as polyimide. The substrate may alsohave conductive traces (not shown) that extend over the top surface 402,the bottom surface 404 and/or between the top and bottom surfaces. Amicroelectronic element 410 such as a semiconductor chip is attached tothe first surface 402 with a flexible substrate 400. The microelectronicelement is electrically interconnected with one or more of theconductive pads 406 using conductive elements 412 such as wire bonds. Anadhesive 414 is preferably used for attaching the microelectronicelement 410 to the substrate 400.

Referring to FIG. 10, the substrate 400 may be placed onto a bottomplate 416 of a mold. The bottom plate 416 preferably includes a topsurface having recesses that are adapted to receive the conductive posts408. The recesses may have a shape that generally mirrors the shape ofthe posts. After the flexible substrate 400 has been positioned atop thebottom plate 416 of the mold, conductive masses 418 such as elongatedconductive elements or solder balls are provided atop the conductivepads 406 (FIG. 9) accessible at the first surface 402 of the flexiblesubstrate 400. A top plate 420 of the mold is then positioned over thebottom plate 416 of the mold for capturing the flexible substrate 400therebetween. Specifically, the top plate 420 of the mold is placed incontact with the first surface 402 of the flexible substrate and thebottom plate 416 of the mold is in contact with the second surface 404of the flexible substrate 400. The mold top plate 420 includes an inlet422 that enables a flowable material to be introduced into a cavity 424defined by the mold bottom plate 416 and the mold top plate 420.

The bottom plate 416 and the top plate 420 of the mold are preferablypressed together to form an air-tight seal at the periphery thereof. Asthe plates are pressed together, the conductive masses 418 areplanarized so that the top surfaces of the conductive masses lie in acommon plane and are the same height above the top surface of the chip410. In addition, the tips of the conductive posts 408 are alsoplanarized so that the tips lie in a common plane and are all at thesame vertical distance relative to the top surface of the chip 410.Although the present invention is not limited by any particular theoryof operation, it is believed that the planarization of the conductivemasses 418 and the conductive posts 408 will provide packages havinguniform vertical heights across the horizontal width thereof. Inaddition, a plurality of packages may be produced, with each packagehaving a substantially identical vertical height. As a result,microelectronic packages having the exact same vertical dimensions maybe quickly and reliably produced.

After the conductive masses 418 and/or the conductive posts 408 havebeen compressed, a curable, flowable material such as a curableencapsulant is introduced into the cavity 424 of the mold through theinlet 422. The curable encapsulant may be clear, opaque or have opticalproperties anywhere along the scale between clear and opaque. Forexample, the encapsulant may be clear when the microelectronic element410 is an LED or optically-sensitive chip. The curable material ispreferably cured to form a cured encapsulant layer, which preferablyprovides stability to the package and protects the microelectronicelement 410, the conductive wires 414 and the conductive masses 418.

Referring to FIG. 11, the cured encapsulant 426 includes a centralsection 428 having a height H₁ that is sufficient to cover thesemiconductor chip 410 and the wire bonds 412. The encapsulant 426 alsoincludes a peripheral region 430 that extends to a peripheral edge 432of the flexible substrate 400. The encapsulant in the peripheral regionhas a height H₂ that is less than H₁. The solder masses 418 include topsurfaces 434 that are exposed at a top surface of the peripheral portion430 of the encapsulant 426. In certain preferred embodiments, theaccessibility of the solder masses 418 is provided by the top mold plate420. As shown in FIG. 10, the height of the cavity 424 in the area ofthe solder masses 418 is less than the height of the cavity overlyingthe chip 410 and the wire bonds 414. As a result, the encapsulant 426does not cover the top surfaces of the solder masses 418 so that thesolder masses are accessible at a top surface of the cured encapsulantlayer 426.

FIG. 12 shows the microelectronic package shown in FIG. 11. The packageincludes the flexible substrate 400 having a first microelectronicelement 410A that is electrically interconnected with one or more of theconductive posts 408 and/or one or more of the conductive masses 418accessible at a peripheral region of the encapsulant 426. Themicroelectronic package also preferably includes a secondmicroelectronic element 410B that is positioned over the firstmicroelectronic element 410A. The second microelectronic element 401B isalso electrically interconnected with one or more of the conductiveposts 408 and/or one or more of the conductive masses 418. As notedabove, a central region 428 of the encapsulant 426 has a heightsufficient to cover the microelectronic elements 410A, 410B and theconductive wires 412 used for electrically interconnecting themicroelectronic elements with the substrate 400. The peripheral region430 of the encapsulant has a lower height so that the tops of theconductive masses 418 are accessible and exposed at an exposed surfaceof the peripheral region 430 and hence, at an exposed surface of thepackage. The structure shown in FIG. 12 provides a microelectronicpackage that may be stacked either on top of or below anothermicroelectronic package.

FIG. 13 shows the microelectronic package of FIG. 12 stacked on top ofanother microelectronic packages. Specifically, a first microelectronicpackage 400A is stacked atop a second microelectronic package 400B,which is stacked atop a third microelectronic package 400C, which isstacked atop a fourth microelectronic package 400D. The fourmicroelectronic packages are preferably electrically interconnected withone another. The conductive posts 408A of the first microelectronicpackage 400A are in contact with the conductive masses 418B of thesecond microelectronic package 400B. During assembly, the conductivemasses 418B are preferably elevated in temperature so as to at leastpartially transform into a molten state so that the conductive posts408A can be at least partially inserted therein. After the connectionsbetween the conductive masses and the posts have been formed, thetemperature of the conductive masses 418B may be lowered so that theconductive masses re-solidify for permanently connecting the conductiveposts 408A and the conductive masses 418B. The electrical connectionsbetween the second microelectronic package 400B and the thirdmicroelectronic package 400C are made in a similar fashion, as are theelectrical interconnections between the third microelectronic package400C and the fourth microelectronic package 400D. Although FIG. 13 showsan assembly including four microelectronic packages stacked one atop theother, the present invention contemplates that any size package of twoor more microelectronic packages may be manufactured. For example, inone embodiment, a stack of five or more microelectronic packages may bepossible. The uppermost or lowermost package in the stack may beconnected to an external element such as a circuit board or a testboard. Before the individual microelectronic packages are assembledtogether in a stack, each individual package is preferably tested.

Referring to FIG. 14, in another preferred embodiment of the presentinvention, a top plate 520 of a mold includes a top surface 536 and abottom surface 538. The mold top plate 520 includes an inlet 522 thatextends into a cavity 524. The mold top plate 520 also includes one ormore projections 540 that extend into the cavity 524.

Referring to FIG. 15, after the substrate 500 has been positioned atopthe mold bottom plate 516, the mold top plate 520 is secured over thesubstrate 500. The projections 540 of the mold top plate 520 preferablycompress the conductive masses 518 that are positioned atop the firstsurface of the substrate 500. The conductive posts may also becompressed. As described above, the compression of the conductive massesand/or the conductive posts serves to planarize the conductive elementsso that the vertical height of the package is uniform across thepackage. After compression, a curable encapsulant material is introducedthrough inlet 522. The curable encapsulant covers the microelectronicelement 510, the conductive wires 512 and the conductive masses 518.

Referring to FIG. 16, after the encapsulant material is cured, thepackage 500 may be removed from the mold. As shown in FIG. 16, theencapsulant 526 covers microelectronic elements 510A, 510B andconductive wires 512. The peripheral portion 530 of the encapsulant 526has a height that is greater than the height of the conductive masses518. As a result, although the conductive masses 518 are recessed withinthe openings, the conductive masses have surfaces which are exposed andaccessible at the top of the encapsulant layer 526.

Referring to FIG. 17, microelectronic packages similar to the one shownin FIG. 16 may be stacked atop one another to form a microelectronicstack. As shown in FIG. 17, a first microelectronic package 500A isstacked atop a second microelectronic package 500B. The conductive posts508A of the first microelectronic package 500A are in electrical contactwith the conductive masses 518B of the second microelectronic package500B. The stacked package has a lower overall silhouette due to therecess of the conductive masses 518 in the encapsulant layer 526.

FIG. 18 shows a top plate 620 of a mold, in accordance with anotherpreferred embodiment of the present invention. The top plate 620includes a cavity having depressions 640 formed therein that arepreferably provided in the same locations as the projections 540 shownin the FIG. 14 embodiment. Referring to FIG. 19, as the conductivemasses are compressed by the mold, the conductive masses 618 extend intothe recesses 640. As a result, when the encapsulant 626 is introducedinto the mold, the upper ends of the conductive masses project above thetop surface 642 of the encapsulant 626.

Referring to FIG. 20, the encapsulant 626 may be cured and the package600 removed from the mold. Due to the recesses 640 in the top plate 620of the mold (FIG. 18), the upper ends of the conductive masses 618project above the top surface 642 of the encapsulant 626 and are exposedat the top surface.

Referring to FIG. 21, two or more of the microelectronic packages 600A,600B shown in FIG. 20 may be stacked atop one another to form amicroelectronic stack. As shown in FIG. 21, the conductive posts 608A ofthe first microelectronic package 600A are in contact with theconductive masses 618B of the second microelectronic package 600B forforming the electrical interconnection. When assembling the stack, theconductive masses 618 are preferably transformed into an at leastpartially molten state so that the conductive post 608 may be insertedtherein. After the temporary connection has been made, the conductivemasses may be transformed back into a solid state for forming apermanent connection between the conductive posts and the conductivemasses. Although only two microelectronic packages are shown in FIG. 21,it is contemplated that the present invention will enable three, four ormore microelectronic packages to be stacked atop one another.

Referring to FIG. 22, in accordance with another preferred embodiment ofthe present invention, a microelectronic package includes a substrate700, such as a flexible substrate made of a dielectric material, havinga first surface 702 and a second surface 704. The package includesconductive posts 708 projecting from the first surface 702 andconductive pads 706 accessible at the second surface 704. The flexiblesubstrate is juxtaposed with a bottom plate 716 of a mold havingrecesses 717 formed therein. A conductive mass 718, such as a solderball, is placed in each of the recesses 717 of the bottom plate 716. Thesubstrate 700 is preferably placed in contact with the bottom plate 716so that each of the conductive pads 706 is in contact with one of theconductive masses 718.

Referring to FIG. 23, a top plate 720 of a mold is preferably placedover the microelectronic package and the bottom plate 716 of the mold.The substrate 700 of the microelectronic package is preferablysandwiched between the mold bottom plate 716 and the mold top plate 720.The mold is compressed for planarizing the tips of the conductive posts708 and the bottoms of the conductive masses 718. The mold may be heatedfor transforming the conductive masses 718 to an at least partiallymolten state for wetting the conductive masses to the conductive pads706. A curable encapsulant material may be introduced through an inlet722 of the top plate 720. The curable encapsulant material preferablyenters the cavity 724 and covers the microelectronic element 710, theconductive wire 712 and surrounds the conductive posts 708, with thetips of the posts being accessible and exposed at an upper surface ofthe encapsulant layer. The curable liquid encapsulant is preferablycured to provide the microelectronic package shown in FIG. 24. Theencapsulant layer may be rigid, at least partially rigid or compliant.The encapsulant 726 preferably has a central region 728 that covers themicroelectronic elements 710A, 710B and the conductive wires 712. Thetips of the conductive posts 708 are preferably accessible through theencapsulant at a peripheral region 730 of the encapsulant 726. Theconductive masses 718 are accessible at a bottom of the microelectronicpackage.

FIG. 25 shows another view of the microelectronic package shown in FIG.24. The microelectronic package includes substrate 700 having a firstsurface 702 and a second surface 704 remote therefrom. The packageincludes a first microelectronic element 710A overlying a secondmicroelectronic element 710B. The microelectronic elements areelectrically interconnected with the substrate using conductive wires712. The package includes encapsulant layer 726 having a central region728 having a height H₁ and a peripheral region 730 having a lower heightH₂. The height H₁ of the central region 728 is sufficient to cover themicroelectronic elements 710A, 710B and the wire bond 712. The lowerheight H₂ of the peripheral region 730 enables the tips of theconductive posts 708 to be accessible therethrough and have portionswhich are exposed at an exposed surface of the peripheral region 730 ofthe encapsulant layer, and hence, at an exposed surface of the package.Thus, the structure shown in FIG. 25 provides a microelectronic packagehaving conductive elements, such as conductive posts, accessible andhaving portions exposed at a top surface thereof and second conductiveelements, such as solder balls, accessible at a bottom surface thereof.As a result, the microelectronic package may be stacked with one or moreother microelectronic packages. In other preferred embodiments, thepackage may have first conductive posts accessible at a top of thepackage and second conductive posts accessible at a bottom of thepackage for stacking.

FIG. 26 shows the microelectronic package of FIG. 25 being stacked atopsimilarly designed microelectronic packages. As shown in FIG. 26, afirst microelectronic package 700A is stacked atop a secondmicroelectronic package 700B, which is stacked atop a thirdmicroelectronic package 700C, which is stacked atop a fourthmicroelectronic package 700D. The electrical interconnections are formedbetween the first and second packages by the conductive masses 718Abeing in contact with the tips of the conductive posts 708B of thesecond microelectronic package 700B. The embodiment shown in FIG. 26shows four microelectronic packages being stacked one atop the other,but it is contemplated that other embodiments may include two, three,four or more microelectronic packages stacked one atop another.

Referring to FIG. 27, in certain preferred embodiments of the presentinvention, a microelectronic package includes a substrate 800, such as aflexible, circuitized substrate, having a first surface 802 and a secondsurface 804. Conductive masses 818, such as solder, are placed atop thefirst surface 802 and conductive pads 806 are accessible at the secondsurface 804. At least some of the conductive masses 818 are preferablyin electrical connection with at least some of the conductive pads 806.

Referring to FIG. 28, a microelectronic element 810 such as asemiconductor chip is attached to the first surface 802 of the substrate800. The microelectronic element 810 is electrically interconnected withone or more of the conductive masses 818 and one or more of theconductive pads 806 via wire bonds 812.

Referring to FIG. 29, the microelectronic package is placed in a moldincluding a bottom plate 816 and a top plate 820 having an inlet 822.The flexible substrate 800 is preferably sandwiched between the bottomplate 816 and the top plate 820. The mold includes a cavity 824extending between the top plate 820 and the bottom plate 816. When themold is closed, an underside of the top plate 820 at least partiallycompresses or collapses the conductive masses 818. As a result, topsurfaces of the compressed conductive masses are preferably planarizedso that they lie in a common plane. A curable encapsulant is preferablyintroduced into the cavity 824 through an inlet 822. The curableencapsulant preferably covers the microelectronic element 810, theconductive wires 812, and surrounds the conductive masses 818.

Referring to FIG. 30, the encapsulant 826 is cured to provide anovermold, whereby the compressed conductive masses 818 have top surfacesthat lie in a common plane and are exposed and accessible at a topsurface of the encapsulant 826. The conductive pads 806 remainaccessible at a bottom of the microelectronic package.

Referring to FIG. 31, a stencil 850 having opening 852 is preferablypositioned atop the encapsulant layer 826 of the microelectronicpackage. The stencil openings 852 are preferably placed in substantialalignment with the conductive masses 818 exposed and accessible at thetop surface of the encapsulant 826. A solder paste 854 may be swept overthe stencil 850 so that a mass of solder paste 856 is deposited atopeach conductive mass 818. In other preferred embodiments, the solderpaste 856 may be deposited atop the conductive masses without using astencil.

Referring to FIG. 32, the conductive masses 818 and the solder paste 856deposited thereon are desirably reflowed to form an elongated conductiveelement that projects above a top surface of the encapsulant layer 826.As shown in FIG. 32, a portion of the conductive element is exposed.Conductive elements 858 such as solder balls are attached to theconductive pads 806 at the bottom of the package.

FIG. 33 shows the compressed or collapsed solder balls 818, which areaccessible at a top surface of the encapsulant layer 826. FIG. 34 showsanother view of the package of FIG. 32 including substrate 800,microelectronic element 810 secured atop substrate 800 and conductivewires 812 for electrically interconnecting the microelectronic element810 and the substrate 800. The package includes encapsulant layer 826and elongated conductive elements projecting above a top surface of theencapsulant layer. The elongated conductive elements comprise thereflowed solder balls and solder paste.

FIG. 35 shows the microelectronic package of FIG. 34, after the packagehas been assembled into a microelectronic stack. The stack desirablyincludes first microelectronic package 800A stacked atop secondmicroelectronic package 800B, which is stacked atop thirdmicroelectronic package 800C, which is stacked atop fourthmicroelectronic package 800D. The conductive pads 806A of the firstmicroelectronic package 800A are in contact with the elongatedconductive elements 857B of the second microelectronic package 800B.Specifically, the conductive pad 806A is in contact with the portion ofthe elongated conductive element 857B that projects above the topsurface of the encapsulant layer 826. As a result, a stackable assemblymay be formed. As shown in FIG. 35, only the bottom microelectronicpackage 800D has solder balls 858 attached to the conductive pads 806Daccessible at the second surface 804D of the fourth package 800D.

FIG. 36 shows an X-ray image after the elongated conductive elements 857have been formed. As shown in FIG. 36, flexible substrate 800 includesfirst surface 802 and second surface 804. Initially, solder balls 818are provided over the first surface 802 and flexible substrate 800. Thesolder balls are compressed by the mold for planarization and theencapsulant layer 826 is introduced into the cavity of the mold forsurrounding the compressed solder balls 818. A solder paste 856 ispreferably deposited atop each of the compressed solder balls 818 usingthe stencil shown and described above in FIG. 31. The compressed solderballs 818 and the solder paste 856 is reflowed for forming the elongatedconductive element 857. FIG. 37A shows a cross-sectional view of theassembly shown in FIG. 36. The microelectronic package includessubstrate 800 with compressed solder balls 818 formed thereon and solderpaste 856 deposited atop each of the compressed solder balls 818. Thesolder balls 818 and the solder paste 856 are reflowed to form theelongated conductive elements 857. The top of the elongated conductiveelement 857 extends above the top surface of the encapsulant layer 826.FIG. 37B shows a magnified view of one of the elongated conductiveelements 857 shown in FIG. 37A.

FIG. 38 shows a microelectronic package in accordance with anotherpreferred embodiment of the present invention. The package includes asubstrate 900 having a top surface 902 and a bottom surface 904 remotetherefrom. A first microelectronic element 910A is attached to the firstsurface 902 of the substrate 900 and a second microelectronic element910B is provided over the first microelectronic element 910A. In otherpreferred embodiments, three or more microelectronic elements may bestacked atop the substrate 900 in a manner similar to the manner shownin FIG. 38. The microelectronic elements are electrically interconnectedwith the substrate using conductive wires 912. The substrate 900includes first conductive pads 906 accessible at the top surface 902 ofthe substrate and second conductive pads 907 accessible at the secondsurface 904 of the substrate. Conductive masses 918A, 918B, such assolder balls, are provided over the respective conductive pads 906, 907.As shown in FIG. 38, first solder balls 918A are provided over theconductive pads 906 accessible at the first surface 902 of the substrateand second solder balls 918B are provided over the conductive pads 907accessible at the second surface 904 of the substrate 900. The packageis desirably placed in a mold similar to one of the molds shown anddescribed above for at least partially compressing/collapsing both thefirst solder balls 918A and the second solder balls 918B. The mold maybe heated for transforming the solder balls into an at least partiallymolten state. The mold preferably planarizes the solder balls so thatthe top surfaces of the first conductive masses 918A lie in a firstcommon plane and the bottom surfaces of the second conductive masses918B lie in a second common plane. Although the present invention is notlimited by any particular theory of operation, it is believed that theplanarization of the conductive masses will enable the mass productionon a plurality of microelectronic packages, each package having astandard height. A curable encapsulant may be injected into the mold forcovering the microelectronic chips 910A, 910B, the conductive wires 912and surrounding the compressed or collapsed conductive masses 918A,918B. The mold is preferably designed so that the top surfaces of thefirst set of collapsed solder balls 918A are accessible and exposed atthe top surface of the encapsulant and the bottom surfaces of the secondset of solder balls 918B are accessible and exposed at the bottomsurface of the encapsulant. The structure shown in FIG. 38 may bestacked atop other microelectronic packages to form a stacked assembly,similar to the stacked assembly shown in FIG. 35. In certain preferredembodiments, one or more of the conductive masses 918A, 918B may bereplaced by a conductive post, such as the posts shown in FIG. 25 of thepresent application.

In certain preferred embodiments of the present invention, amicroelectronic package, assembly or stack may include one or morefeatures of one or more of the embodiments disclosed in U.S. applicationSer. No. 10/959,465, filed Oct. 6, 2004, entitled “Formation ofCircuitry With Modification of Feature Height” [TESSERA 3.0-358]; U.S.application Ser. No. 11/166,861, filed Jun. 24, 2005, entitled“Structure With Spherical Contact Pins” [TESSERA 3.0-416]; U.S.application Ser. No. 11/014,439, filed Dec. 16, 2004 [TESSERA 3.0-374],claiming priority of U.S. Provisional Application Ser. No. 60/533,210,filed Dec. 30, 2003; U.S. application Ser. No. 10/985,126, filed Nov.10, 2004 [TESSERA 3.0-375], claiming priority of U.S. ProvisionalApplication Ser. No. 60/533,393, filed Dec. 30, 2003; U.S. applicationSer. No. 10/985,119, filed Nov. 10, 2004; [TESSERA 3.0-376], claimingpriority of U.S. Provisional Application Ser. No. 60/533,437, filed Dec.30, 2003; U.S. patent application Ser. No. 11/140,312, filed May 27,2005 [TESSERA 3.0-415], claiming priority of U.S. ProvisionalApplication Ser. No. 60/583,066, filed Jun. 25, 2004 and U.S.Provisional Application Ser. No. 60/621,865, filed Oct. 25, 2004; U.S.Provisional Application Ser. No. 60/662,199, filed Mar. 16, 2005[TESSERA 3.8-429]; U.S. Patent Application Publication No. 2005/0035440[TESSERA 3.0-307]; and U.S. Provisional Application Ser. No. 60/753,605,filed Dec. 23, 2005, entitled “MICROELECTRONIC PACKAGES AND METHODSTHEREFORE” and assigned attorney docket number TESSERA 3.8-482, thedisclosures of which are hereby incorporated by reference herein.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method of making a microelectronic assembly comprising: providing a microelectronic package including a substrate, a microelectronic element overlying said substrate and at least two conductive elements projecting from a surface of said substrate, first and second conductive elements of said at least two conductive elements having surfaces remote from said surface of said substrate and being electrically connected through said substrate to said microelectronic element for carrying a first signal electric potential on said first conductive element and for simultaneously carrying a second electric potential on said second conductive element, said second electric potential being different from said first signal electric potential; molding a dielectric material around said at least two conductive elements for supporting said microelectronic package and so that said remote surfaces of said at least two conductive elements remain accessible and exposed at an exterior surface of said molded dielectric material and said remote surfaces are at least substantially planar and project to heights from said surface of said substrate greater than a height of the exterior surface of the molded dielectric material from said surface of said substrate.
 2. The method as claimed in claim 1, wherein said molded dielectric material surrounds at least portions of individual ones of said at least two conductive elements at heights lower than said height of the exterior surface of the molded dielectric material.
 3. The method as claimed in claim 1, wherein said at least two conductive elements project from a first surface of said substrate and said substrate further comprises a second surface remote from said first surface and at least a third conductive element and a fourth conductive element projecting from said second surface of said substrate, said at least third and fourth conductive elements having surfaces remote from said second surface of said substrate and being electrically connected through said substrate to said microelectronic element for carrying a third signal electric potential on said third conductive element and for simultaneously carrying a fourth electric potential on said fourth conductive element, said fourth electric potential being different from said third signal electric potential.
 4. The method as claimed in claim 3, further comprising compressing said at least third and fourth conductive elements so that said remote surfaces of said at least two second conductive elements lie in a common plane.
 5. The method as claimed in claim 3, wherein said substrate is flexible.
 6. The method as claimed in claim 3, wherein said substrate comprises a polymeric material.
 7. The method as claimed in claim 3, wherein said microelectronic element is a semiconductor chip having a front face and contacts at said front face, and a back face remote therefrom, said front face facing said substrate.
 8. The method as claimed in claim 3, wherein said microelectronic element is a semiconductor chip having a front face and contacts at said front face, and a back face remote therefrom, said back face facing said substrate.
 9. The method as claimed in claim 3, further comprising a compliant layer disposed between said microelectronic element and said substrate.
 10. The method as claimed in claim 3, wherein at least some of said conductive elements and said microelectronic element overlie a first surface of said substrate.
 11. The method as claimed in claim 3, wherein said microelectronic element overlies a first surface of said substrate and at least some of said conductive elements overlie a second surface of said substrate remote opposite said first surface.
 12. The method as claimed in claim 3, wherein said substrate includes a plurality of dielectric layers and conductive traces extending along said dielectric layers.
 13. The method as claimed in claim 3, wherein said microelectronic package is a first microelectronic package, further comprising making a second microelectronic package using the method of claim 3, and stacking said second microelectronic package atop said first microelectronic package, wherein said first and second microelectronic packages are electrically interconnected together through said at least two conductive elements of each of said first and second microelectronic packages.
 14. The method as claimed in claim 3, wherein said molding includes flowing an encapsulant material into a cavity of a mold overlying said substrate and said conductive elements, and curing said encapsulant material. 